Sams Teach Yourself C in 21 Days

Working with the Screen, Printer, and Keyboard 371

14

error_message(“An error has occurred.”);

void error_message(char *msg)

{

fprintf(stderr, msg);

}

By using your own function instead of directly calling fprintf(), you provide additional

flexibility (one of the advantages of structured programming). For example, in special

circumstances you might want a program’s error messages to go to the printer or a disk

file. All you need to do is modify the error_message()function so that the output is

sent to the desired destination.

Printer Output Under DOS and the Windows Command Prompt

On a DOS or Windows system, you send output to your printer by accessing the prede-

fined stream stdprn. On IBM PCs and compatibles, the stream stdprnis connected to

the device LPT1: (the first parallel printer port). Listing 14.15 presents a simple example.

To usestdprn, you need to turn ANSI compatibility off in your compiler.

Consult your compiler’s manuals for more information. Because stdprnis not

a part of the ANSI standard, your compiler may not support it.

Note

LISTING14.15 printer.c. sending output to the printer

1: /* Demonstrates printer output. */

2: /* stdprn is not ANSI standard */

3: #include <stdio.h>

4:

5: int main( void )

6: {

7: float f = 2.0134;

8:

9: fprintf(stdprn, “\nThis message is printed.\r\n”);

10: fprintf(stdprn, “And now some numbers:\r\n”);

11: fprintf(stdprn, “The square of %f is %f.”, f, f*f);

12:

13: /* Send a form feed. */

14: fprintf(stdprn, “\f”);

15:

16: return 0;

17: }

This message is printed.

And now some numbers:

The square of 2.013400 is 4.053780.

OUTPUT

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